Z-parameter Method For Life Prediction And Its Application Of Heat-resistant Steels

Many engineering components and structures are subjected to high temperatures and pressures simultaneously, especially in thermal power plants, boilers, petroleum/petrochemical and other process industries. Their mechanical characteristics are subjected to deterioration during long-term service exposure due to microstructual degradation and the accumulation of damage. These components and structures will come to failure finally and creep is the main factor. Creep life assessment and damage evaluation of high-temperature materials are major concern because of safety and economic reasons.A Z-parameter method is proposed based on the study of life assessment and damage evaluation strategy of major heat-resistant steels. This paper is mainly concerned with reliability analysis of creep rupture property, life prediction and damage evaluation of heat-resistant steels based on the Z-parameter method, such as HK40,5Cr-0.5Mo,GH145 and 30Cr1Mo1V steels which are widely used in petrochemical and steam boiler plants. The results show that:1. The Z-parameter method can characterize the reliability evaluation of creep rupture life. For a material with the same damage state, the value of Z-parameter reflects the deviation of data point from the master curve. Therefore, the distribution of Z-parameter can represent the dispersibility of creep rupture data. Because of the inherent dispersibility of material, the predicted results are connected with the reliability. Damage evaluation combined with the Robinson's rule has been performed by introducing the confidence level according to the creep rupture data both from new and service tubes. The findings indicate that the values of creep rupture life and damage accumulation are related to the confidence level.2. The mean value of Z-parameter method can represent the damage accumulation of material with different damage state. Comparing the curves of creep rupture data with the values of Z of different segments and different service time, it is well proved that there exists a close correlation between the value of the Z-parameter and the deterioration of creep rupture properties. The three stages in the relationship between Z and service time may demonstrate a rapid decrease of rupture property at early service stage, and a following relative long stable stage; when microstructure degradation develops to a certain degree, serious deterioration of rupture property will take place.